The Coulombi Egg Oil Tanker - Ballast Tanks - Key to Safety
Better protection, safer and more economical than Double Hull

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COULOMBI EGG oil tanker design - the key to safety is the ballast tanks.

A Double Hull VLCC has about 230 000 m² of surface area in the tank body double hull segregated ballast tanks. In a rare accident when the tanker is loaded, the double hull/ballast tanks shall prevent discharge of oil into the environment, but as the tanker is in ballast almost half of its life, it is even more important to protect the ballast tanks against corrosion, so the structure will not waste away, making the VLCC unsafe and a risk of pollution due to structural defects, wear and tear.

The corrosion protection may consist of two coats of light colored tar free epoxy paint and its life may be between 2 - 10 years depending on the quality of the application. Minor imperfections in application may be protected by about 2 300 zinc anodes (about 50 tonnes of zinc). First cost may be of the order US$ 4.0 million. Some 1996 built double hull VLCC's had only one coat (to save first cost?) and today (December 2002) the coating has already broken down and corrosion and serious problems have started.

The only solution is re-coating but how to do that? Future repairs and maintenance of the coating are not easy, as access and space inside the double hull are limited by hundreds of manholes and the areas involved are very large.

No doubt the coating will first break down in the highly stressed areas, local corrosion will start, fatigue or corrosion cracks develop and then anything can happen, e.g. the side shell falls off with a massive oil spill as a consequence. One weakness of Double Hull is that the outer side shell supports the inner shell and vice versa and that fractures in one may cause collapse of the two.

Latest findings show that Double Hull crude oil tankers also need coating protection against pitting and corrosion in the bottom of the cargo tanks! The reason for this is multiple: sulphuric acid or biological matters concentrated in the water in certain crude oils that accumulates at the bottom, high temperature inside the double hull heating up the cargo oil, elastic deformation of the inner bottom plate up to 10 mm between stifferners and floors, crystaline modifications of the steel in way of welding butts and seams, etc. With no protection the result has been catastrophic - the pitting rate may be 2-3 mm per year on the cargo side, after three, four years the pitting has eaten through half the bottom plate, which then splits - oil fills the double bottom! Thus you have to add 15 000-20 000 m² of sophisticated coating of the cargo tanks bottom to the above in a double hull VLCC.

The COULOMBI EGG tanker simplifies the problem in an innovative way - the double hull ballast tanks are replaced by top side tanks only, which, according to the IMO/MEPC Marpol I/13F(5) definition of tanker marine environment protection, provide equivalent protection of the marine environment as Double Hull. But the structural area inside the ballast tanks is reduced more than three times (to abt. 66 000 m² in a VLCC) and the first cost of corrosion protection is reduced in proportion. Future repairs and maintenance of the coating are possible and much easier thanks to much better and easier access and more space.

No doubt the risk of structural failure, wear and tear in the ballast space, and therefore also the risk of leaks of cargo and hydrocarbon gas into the ballast space, explosions and fires, are reduced - and the safety is improved. The single hull cargo bottom plate does not need any inside corrosion or pitting protection based on single hull experience.


When 1000's of tonnes of muck and dirt may collect in the bottom of a Double Hull Suezmax tanker or VLCC from ballast taken in rivers and ports, all muck and dirt are always flushed out when deballasting a COULOMBI EGG tanker by gravity, as it is carried above the ballast water line (see above figure of a COULOMBI EGG tanker midship section).

When a COULOMBI EGG tanker arrives to load cargo, all ballast water is discharged by gravity (through valves in the ship's side). It could easily have been replaced during the voyage to avoid spreading alien marine species - another IMO concern. In port cargo is loaded simultaneously with deballasting and no ballast pump need be started during the operation.

Contact anders.bjorkman@wanadoo.fr